Field of the Invention
The invention relates to a drill bit, in particular a bit for drilling rock, comprising an elongated shank and at least one cutting insert at one end, this cutting insert having at least one front-end cutting edge running from the circumference to the rotary axis of the drill bit. The cutting edge is formed by virtue of the fact that a rake face, lying in front of the cutting edge in the direction of rotation of the drill bit, and a flank, lying behind the cutting edge in the direction of rotation of the drill bit, adjoin one another, and the rake face and the flank enclose a wedge angle xcex3 with one another.
Drill bits for drilling rock generally consist of a cylindrical steel shank with helically twisted flutes for removing the drillings, with a clamping end to be accommodated in the drilling machine and with a drill bit head at the other end. The drill bit head being formed by one or more cutting inserts connected to the steel shank as a rule by brazing and made of a wear-resistant material, such as cemented carbide.
The cutting inserts, due to the rotary movement of the drill bit and as a rule due to an additional axial hammer movement superimposed on the rotary movement, are subjected to abrasion stress on the one hand and to percussion stress on the other hand.
The requirement for such drill bits, in addition to as long a service life as possible, is to also achieve as good a drilling capacity as possible (drilling advance per unit of time).
The most common, conventional drill bits for drilling rock have as drill bit head, a cutting insert with two front-end cutting edges which adjoin one another in a roof shape and which have a uniform wedge angle along its profile from the circumference to the center of the drill bit, this wedge angle as a rule lying within a range of 80xc2x0 to 100xc2x0. Although this relatively large wedge angle results in high stability and resistance to chipping of the cutting edge, the chiseling action, achieved in particular by the hammer movement of the drill bit, and thus the drilling advance per unit of time, that is to say the drilling capacity, are not optimal.
The pertinent art, therefore, does not lack a showing of prior attempts to also increase in particular the drilling capacity. Thus, by way of example, Austrian Utility Model 4.252 describes a drill bit for drilling rock having a cutting insert wherein the clearance angle xcex2 of the cutting edge increases with increasing distance from the rotary axis D and the wedge angle xcex3 correspondingly decreases.
An improvement in the drilling capacity is certainly achieved by such a design. However, the smallest wedge angle xcex3 at the outer circumference of the cutting insert must not become too small in order to avoid chipping of the cutting edges at this point, so that the drilling capacity is still not optimal even in such a drill bit.
German published patent application DE 199 44 406 A1 describes a rock drill bit wherein the flank extends into the cutting insert in a manner differing from a flat profile or is concave.
In this way, the wedge angle xcex3 is certainly reduced and thus the drilling capacity improved. However, if the entire flank is designed in such a way, chipping of the cutting edges may occur, in particular at the marginal section of the cutting edge. In order to prevent this, is it advantageous according to this pre-publication to provide that region of the flank which directly adjoins the cutting edge with a protective chamfer having a comparatively larger wedge angle, which makes the drilling capacity slightly worse again. In addition, a high frictional stress with the material to be removed occurs in such a drill bit due to the flat rake face, a factor which again causes premature wear of the cutting insert. Furthermore, the cutting capacity when cutting steel, which is of importance in particular when drilling through reinforcements in concrete, is poor.
It is accordingly an object of the invention to provide a drill bit for drilling rock, which overcomes the above-mentioned disadvantages of the heretofore-known devices of this general type and which has a good drilling capacity, with good resistance to chipping of the cutting edges, and good wear resistance. In addition, the cutting of iron materials is also to be readily possible.
With the foregoing and other objects in view there is provided, in accordance with the invention, a drill bit, in particular for drilling rock, comprising:
an elongated shank defining a rotary axis D of the drill bit and having at least one cutting insert at an end thereof;
the cutting insert having at least one front-end cutting edge extending from a periphery to the rotary axis D of the drill bit, the cutting edge being formed by a rake face, lying ahead of the cutting edge in a direction of rotation of the drill bit, and a flank, lying behind the cutting edge in the direction of rotation of the drill bit, the rake face and the flank face adjoining one another and enclosing a wedge angle xcex3 with one another;
the rake face having formed therein at least one recess between a marginal segment at the periphery of the drill bit and a center segment at a center of the drill bit, the recess running into the adjacent the flank and reducing the wedge angle xcex3 defining the cutting edge at least in a longitudinal section extending directly up to the cutting edge, relative to the wedge angle xcex3 at the remaining the cutting-edge segments.
In other words, the objects are achieved with the invention, in that the rake face, between a marginal section at the circumference of the drill bit and a section at the center of the drill bit, has one or more recesses which run right into the adjacent flank and which reduce the wedge angle xcex3, at least in a longitudinal section extending directly up to the cutting edge, relative to the remaining cutting-edge sections.
Due to the recesses in the rake face, a reduction in the wedge angle xcex3 and thus a better drilling capacity of the drill bit are achieved. Owing to the fact that a rake-face section at the center and at the margin of the drill bit is designed without these recesses, the stability of the cutting edge is increased in these extremely sensitive zones. Thus the recesses in the sections lying in between can extend without a supporting chamfer up to the cutting edge and run into the adjacent flank, so that an extremely favorable chiseling action occurs as a result without having to fear chipping of the cutting edges. Since the rake-face section at the center of the drill bit has no recesses, and the recesses of the rake face in the adjoining region run right into the adjacent flank, the cutting-edge sections at the drill bit point stand out slightly from the adjacent cutting-edge sections and a good centering effect of the drill bit is achieved. Owing to the fact that the recesses are made in the rake face, a type of chip breaker groove is also obtained, which improves the disposal of the removed material and reduces the friction effect. As a result, the wear on the drill bit is markedly reduced. In addition, due to the virtually continuously very sharp cutting edges in this way, a very good cutting action is achieved even when cutting steel, and the chips running off due to the rotary movement are effectively deflected or discharged on account of the recesses.
It has proved to be especially successful if the wedge angle xcex31 reduced by the recesses lies within a range of between 50xc2x0 and 70xc2x0, while the wedge angle xcex3 along the remaining cutting-edge sections lies within the range of between 80xc2x0 and 100xc2x0.
For the optimum stability and resistance to chipping of the cutting edges, it has proved successful to design the cutting inserts in such a way that the length y of the marginal section at the circumference of the drill bit lies within a range of about 0.37 to 0.45 (37-45%) of the cutting-tip thickness, and the length x of the section at the center of the drill bit lies within a range of about 0.09 to 0.11 (9-11%) of the drill bit diameter.
In particular in the case of relatively large drill bit diameters, it is advantageous to provide not only one but also two or even more recesses between the center section and the marginal section in the cutting insert or cutting inserts of the drill bit. The length z of the rake-face sections having a large wedge angle xcex3 between the individual recesses is then to preferably lie within a range of about 0.07 to 0.13 (7-13%) of the cutting-tip thickness. Thus, optimum stability of the cutting edges is also obtained in the case of large drill bit diameters.
Furthermore, it has proved to be advantageous to design the recesses with a flat center region with rounded portions running out laterally and downward, the rake angle xcex1 in the flat center region of the recesses being about 90xc2x0.
It may also be advantageous for the disposal of the drillings or shavings to provide the recesses in such a way that they merge from the cutting insert right into the end section of the drill bit shank for accommodating the cutting insert and only run out there into the helical flutes for removing the drillings.
Especially good disposal of the removed drillings is made possible in this way.
If the drill bit is designed with a one-piece cutting insert having two front-end cutting edges running toward one another in a roof shape, it has proved successful to have the cutting edges run toward one another at a tip angle xcex4 of about 130xc2x0.
Furthermore, it is advantageous if the two front-end cutting edges of the drill bit which run toward one another in a roof shape are each divided into two different sections, and if the respectively adjoining central cutting-edge sections enclose an tip angle xcex41 of about 130xc2x0 with one another, and the two circumferential cutting-edge sections enclose an tip angle xcex42 of more than 130xc2x0, preferably 150xc2x0, with one another. This achieves the effect that the region of the cutting edge which is subjected to the greatest wear is specifically strengthened by additional material accumulation.
With the above and other objects in view there is also provided, in accordance with the invention, a cutting insert for a drill bit, in particular the above-summarized drill bit, comprising:
a cutting insert body formed with a rotary axis D and a periphery;
the cutting insert body being formed with at least one front-end cutting edge extending from the periphery to the rotary axis D;
a rake face formed on the cutting insert body ahead of the cutting edge in a direction of rotation of the cutting insert body, and a flank behind the cutting edge in the direction of rotation, the rake face and the flank face adjoining one another along the front-end cutting edge and enclosing a wedge angle xcex3 with one another;
the rake face having formed therein at least one recess between a marginal segment at the periphery and a center segment at a center of the cutting insert body, the recess reducing the wedge angle xcex3 defining the cutting edge, at least in a longitudinal section extending directly up to the cutting edge, relative to the wedge angle xcex3 at remaining the cutting-edge segments.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a drill bit for drilling rock, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.